1. Field of the Invention
The present invention relates to a dispersion compensation method and a compensation node apparatus, and, in particular, to a dispersion compensation method and a compensation node apparatus for compensating chromatic dispersion occurring in an optical fiber in a wavelength division multiplexing transmission system.
2. Description of the Related Art
In order to respond to a recent increase in communication network usage, research and development for achieving increase in a transmission capacity and a transmission distance of an optical communication system have been proceeded with. Currently, a wavelength division multiplexing (WDM) optical transmission system has been put into a practical use at a base transmission rate of 10 Gbit/s per channel. Further, a study of an optical transmission system of 40 Gbit/s in a next generation has been proceeded with for the purpose of further cost reduction. Furthermore, recently, a multi-function photonic network is demanded, and, not only a function of point-to-point transmission but also a function of switching an optical signal route freely by means of an optical add/drop multiplexing (OADM) or an optical cross-connect (OXC) is required.
However, for a high-bit-rate optical transmission more than 10 Gbit/s, optical waveform degradation due to a so-called ‘chromatic dispersion’ in which a propagation speed of light becomes different for each wavelength becomes a serious problem. Further, since a chromatic dispersion tolerance becomes strict in inverse proportion to a square of bit rate of a signal to transmit, the wavelength tolerance for a signal of 10 Gbit/s is approximately 1000 ps/nm, while the wavelength tolerance for a signal of 40 Gbit/s is approximately 70 ps/nm.
Such a strictness in the tolerance of chromatic dispersion requires limitation of a transmission distance in a dispersion fiber (+17 ps/nm/km) to approximately 60 km for 10 Gbit/s, and approximately 4 km for 40 Gbit/s. In order to solve this problem and to achieve long distance transmission, a dispersion compensation fiber (DCF) having opposite-sign dispersion with respect to that of a channel is in transmission fiber applied as shown in FIG. 1 in the related art.
In FIG. 1, a dispersion compensation fiber is disposed in each of a transmitting end (DCT) 10, an in-line repeater (DCL) 11, a receiving end (DCR) 12 and a compensation node (CN) 13. The compensation node 13 is provided, for example, for each six spans, for the purpose of OADM, OXC or gain equalization.
FIG. 2 shows an example of a dispersion map in a condition of a single bit rate (40 Gbit/s), which has been studied in the related art (see Japanese Laid-open Patent Application No. 2003-298516, for example). In this example, a transmission distance is 600 km, which corresponds to a distance between compensation nodes (6 spans) in FIG. 1. A dispersion deviation amount (channel dispersion amount for one span+dispersion amount in the in-line DCF) is defined as ΔDL. In order to simplify a configuration of a transmitting end, a dispersion compensation amount at transmitter side DDCT is fixed at zero.
FIG. 3A shows Q penalty, which is a degradation amount in a Q value indicating waveform characteristics with respect to a residual dispersion amount (total dispersion amount in the channel+the dispersion compensators) in a case where an in-line dispersion compensation amount DDCL=100% (complete compensation from a dispersion amount for one span of the channel) and 114% (over compensation by 14%) for transmission of 600 km with an SMF (conventional single mode fiber) at 40 Gbit/s. As shown in FIGS. 3B and 3C, it is seen that waveform degradation is smaller and thus, the Q penalty is smaller in the case of 114% over compensation as shown in an eye pattern of FIG. 3C than that in the case of 100% compensation as shown in an eye pattern of FIG. 3B. It is noted that in either case of the in-line dispersion compensation amount DDCL, the dispersion compensation amount at the receiving end (DDCR+VDC) should be set and adjusted shorter so that the residual dispersion should be adjusted to approximately zero.
FIG. 4 shows Q penalty characteristics with respect to a dispersion deviation amount ΔDL for each in-line span (distance between the in-line repeaters) in a case where the dispersion compensation amount (DDCR) at the receiving end is adjusted to approximately zero so that the residual dispersion may become zero in the SMF 600 km transmission (100 km×6 spans). It is seen therefrom that the penalty becomes smaller as the in-line dispersion compensation residual amount is set minus so that over-compensation is carried out by the in-line repeater DCF. FIG. 5 shows an optimum dispersion map based on this result. In FIG. 5, a compensation coefficient in the in-line compensator is set as 1+β=110% and a compensation coefficient in the last stage is set as being 50%, and thus, a total residual dispersion is adjusted at zero.
Further, a demand has been increased to apply an optical add/drop multiplexing apparatus (OADM) such as that shown in FIG. 6A or an optical cross-connect (OXC) such as that shown in FIG. 6B in a channel. In the OADM currently used, once all the channels of signals are separated by means of a demultiplexer, desired channels of signals are branched off externally, or, after desired channels of signals are inserted, all the channels of signals are multiplexed by means of a multiplexer and are sent to a transmission fiber. Further, a configuration called AOTF employing a wavelength selection switch may also be considered. By disposing dispersion compensators (DCF) at a position A immediately before the demultiplexer and a position B immediately subsequent to the multiplexer as shown in FIG. 6A, dispersion compensation for all the channels can be achieved in a lump by means of one or two dispersion compensators. In contrast thereto, it is not preferable to dispose dispersion compensators at positions C immediately subsequent to the demultiplexer or at positions D immediately before the multiplexer in FIG. 6A since the number of dispersion compensators corresponds to the number of all the channels are required in this case and thus a problem may occur in terms of costs or a size of the entire system. Therefore, at a position E in FIG. 6A at which the demultiplexer and the multiplexer are directly coupled together in the optical add/drop multiplexing apparatus, residual dispersion should be controlled to be less than a dispersion tolerance.
FIG. 7 shows a dispersion map in a case where the OADM is disposed, studied in the related art, in consideration of such points (see Japanese Laid-open Patent Application No. 2003-318825, for example). With respect to an optimum residual dispersion for a maximum transmission distance (for example, 3000 km at 10 Gbit/s), OADM is carried out on a line connecting between the maximum distance point and a zero point (0 km, 0 ps/nm). DCFs are disposed separately at a pre-stage and a post stage of a WDM transmission part (at a DCR and a DCT), and are not disposed at each particular channel which is added or dropped in the OADM. At this time, residual dispersion at the position of the OADM is controlled to lie within a dispersion tolerance. Thereby, the number of DCFs can be minimized, and also, a dispersion compensation disposing manner is common among the respective spans (between a transmission end and the CN, between the CNs, and between the CN and the receiving end). Accordingly, equal transmission characteristics can be obtained for each equal transmission distance.
Other than that, Japanese Laid-open Patent Applications Nos. 2000-236299, 2001-339345, 2002-57622, 2002-77053, 11-68657, 11-88261, 8-321805 and 11-331074, for example, disclose methods for compensating chromatic dispersion in a channel. Further, ‘Design of Nx40 Gbit/s multi-terabit/s transmission systems assisted by simple analytic tools’ by Sebastien Bigo, International meeting OAA 2003, held on July 2003, Paper No. WA3, pages 220-222, and ‘Numerical optimization of pre- and in-line dispersion compensation in dispersion-managed systems at 40 Gbit/s’ by Yann Frignac, Jean-christophe Antona, International meeting OFC 2002, held on March 2002, Paper No. ThFF5, pages 612-613 disclose formulas concerning dispersion compensation.